#148. 【NOIP2015】跳石头 （二分答案，最大化最小值）

#include<cstdio>
#include<algorithm>

//#include<bits/stdc++.h>
using namespace std;
#define LL long long
#define pb push_back
#define X first
#define Y second
#define cl(a,b) memset(a,b,sizeof(a))
typedef pair<long long ,long long > P;
const int maxn=500005;
const LL inf=1LL<<45;
const LL mod=1e9+7;

LL a[maxn];
int n,m;
LL L;
bool ok(int d){
int last=0;
for(int i=1;i<n+2-m;i++){
int cur=last+1;
while(cur<=n+1&&a[last]+d>=a[cur])cur++;
if(cur>n+1)return false;
last=cur;
}
return true;
}
void solve(){
int l=0,r=999999999;
while(r-l>1){
int mid=(l+r)/2;
if(ok(mid))l=mid;
else r=mid;
}
printf("%d\n",r);
}
int main(){
while(~scanf("%lld%d%d",&L,&n,&m)){
for(int i=1;i<=n;i++){
scanf("%lld",&a[i]);
}
a[0]=0;a[n+1]=L;
solve();
}
return 0;
}